As an emerging renewable green energy source, solar energy has become a research hotspot. With the rapid development of solar cell industry, polycrystalline silicon which is of low cost and suitable for large-scale production has become one of the leading photovoltaic materials in the art, and is gradually replacing the dominance of the traditional silicon produced by Czochralski process in the solar cell materials market.
Currently, the preparation of polycrystalline silicon ingot mainly adopts directional solidification system method (referred to as DSS) and crystal growth furnace technique provided by GT Solar Company. The method generally comprises heating, melting, solidifying and growing crystals, annealing and cooling steps, etc. In the solidification process of crystal growth, accompanied by the continuous cooling of the bottom of the crucible, the melt silicon nucleated randomly and spontaneously, and then the random nucleation gradually grows. However, since the initial nucleation has not been controlled, the nucleation process is prone to generate dislocations, resulting in disordered crystalline orientations and nonuniform crystalline grains. Thus the polycrystalline silicon ingot obtained by this method is of low quality. The photoelectric conversion efficiency of the solar cell produced by such polycrystalline silicon ingot is relatively low. Therefore, in order to obtain a polycrystalline silicon ingot with low dislocation density, high-quality and less defects, it is very important to provide a DSS method for efficiently creating good initial nucleation of polycrystalline silicon ingot.